Blender Guru Tutorial part 5: coffee cup bottom gets deformed
I'm guessing you're making the the cup after making the donut.
When you did the icing (frosting) on the donut, you will have set the Proportional Editing Falloff to Random to create that uneven edge of the icing running down the side of the donut.

As I recall, I made the same mistake, because Andrew (Blender Guru) omits to tell you to switch off proportional editing (probably because it's a series of videos that he made at different times).
So before scaling the cup, make sure that Proportional Editing  is switched to Disable.